Gamma glutamyl transferase appears in high levels in mammary tissue at parturition and is high in milk throughout lactation. It is at a low level throughout pregnancy and disappears with involution. Prolactin induces the formation of the transferase in explants of mammary tissue of the pregnant mouse. The transferase appears to be a secretory enzyme concerned with the formation and release of milk and probably has a similar function in other secretory systems. The mammary transferase is to be characterized and the mechanism of action determined. Human milk and pregnant mice are used in the studies. High levels of transferase activity are found in mammary growths, benign or malignant, in cyst fluids and in many other tumors derived from secretory epithelial tissue.